Indicator elements or timing elements allow devices such as ink jet print heads to be accurately positioned in space. In general, timing control elements are either rotatable about a central axis, i.e., timing disk, or are movable in a linear direction, i.e., timing rule. Light, projected by a transmitter, passes through the control element, and is intercepted by the receiver. The receiver, responsive to the light, converts the light into an electrical signal capable of controlling machinery and other servo-mechanical devices.
Timing control elements typically are encoded with a selected window pattern, i.e., they have an annular or linear array of windows that alternate in a transparent window, opaque window, transparent window, opaque window pattern. While the transparent window openings allow the transmitted light to pass through the timing disk or rule, the opaque windows prevent the light from passing through the timing disk or rule.
Timing disks as a rule are fixed to a rotating shaft by means of a hub. For linear systems, timing rules are arranged at right angles to a source of light and the associated receiver generates an electrical signal in response to the incoming light. This particular application is used, for example, to control the feeding action of machine tools.
As the timing disk rotates or the timing rule moves in a linear direction, light is directed at the selected window pattern. Because of the window pattern, the transmitted light can only pass through a transparent window. In response to the light, the receiver generates an electrical signal.
The electrical signals serve to establish a control surface for the measurement of rotational speed, acceleration and more accurate positioning of servomechanical elements, as for example a printing head, a robot arm or a tool carrier.
Timing control elements can be made of glass, metal or plastic, however, plastic and metal are typically used in mass production applications. They are produced, for example, in the case of angle indicators or encoding units, e.g. ink jet printers, out of transparent films.
Known timing devices utilize an arrangement whereby the transmitter is placed on one side of the timing structure and the receiver is placed on the other side of the timing structure to capture the light as it passes through the disk. This arrangement has been known to cause a number of problems, including: a requirement for a complex electromechanical apparatus, increased mechanical stress caused by oscillating loads, a larger footprint size for the timing device, and dirt forming on the timing structure, thereby preventing light from passing efficiently through the structure.
U.S. Pat. No. 4,387,374 (Wiener) discloses a timing device in which the indicator device is an operator rotatable cylindrically shaped encoder wheel with longitudinal slits. LED's are used as the light source on the outside of the cylinder and the detector is on the inside of the cylinder and receives light as the cylinder spins and lets light into the center of the cylinder through the slits. While this arrangement allows the timing device to be made smaller, it would be beneficial to decouple the light source from the detector, making the system more flexible.
U.S. Pat. No. 4,953,933 (Asmar) discloses the use of optical fibers or light guides that function as a read-head for such optical position encoders delivering light to a detector to form a timing device. Although decoupling the light source from the detector saves space and allows the timing devices to be used in different applications, the optical fibers placement would be have to be extremely precise in order to deliver a clear signal to the detector resulting in a very complex and expensive timing device.
U.S. Pat. No. 6,201,239 (Yamamoto et al.) discloses an optical encoder that has a surface emitting semiconductor laser as a light source, a movable scale, and a detector. The object of the invention was to provide an optical encoder using a surface emitting laser, wherein when the light source and the scale (patterned) are situated relatively close to each other, such that the scale pitch can be made less than that in a conventional optical encoder. While this reduces the size of the timing device enabling the movable patterned scale and the light source to be close in proximity, the light source and the patterning layer are two separate layers and must be very precisely aligned, increasing the complexity of the timing device. It would be beneficial to be able to decouple the light source and the patterning layer, making the indicator device capable of more flexible setup positions for a variety of different applications and be smaller.